Safety and Efficacy of Monthly Long-acting IM Injection of 25mg or 40 mg GA Depot in Subjects With PPMS

* 30 Subjects with a diagnosis of primary progressive multiple sclerosis (PPMS) who are not treated for PPMS at study entry (except for symptoms relief). * Study product is GA long-acting injection (GA Depot) which is a combination of extended-release microspheres for injection and diluent (water for injection) for parenteral use. GA Depot will be administered intramuscularly (IM). * The study duration for an individual subject in the core study will be 156 weeks, consisting of 4 weeks of screening evaluation (weeks -4 to 0), followed by a 148-week open-label treatment period, and a 4 weeks follow up period: through a total of 41 visits. * Vital signs and safety assessment will be performed at each visit during the study. * Physical examination will be performed at screening, baseline, 1 week after the second GA Depot treatment, 3 months after first GA Depot treatment and every 3 months thereafter. Last physical examination will be performed at FU visit. * MRI will be performed at screenings and every 6 months thereafter until the end of the treatment period . * Safety laboratory tests will be performed at screening, baseline, 1 month after first treatment, and every 3 months thereafter. * Neurological assessment will be performed at screening, baseline, 3 months, and then every 3 months until end of treatment.